In optical communication systems, messages are transmitted typically through optical fibers by carrier waves of optical frequencies that are generated by sources such as lasers or light emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over other communication systems, such as having a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages.
As the development of optical circuits proceeded, it became necessary to have optical waveguide devices which could couple, divide, switch and modulate the optical waves from one optical fiber to another, or from one waveguide device to another. For example devices see U.S. Pat. Nos. 3,689,264, 4,609,252, 4,637,681, and 4,883,743, among others.
The preferred devices are made by photoforming the waveguides within photohardened polymeric structures, which however in harsh environments suffer from sensitivity to environmental gases, such as moisture in high humidity atmospheric conditions.
Thus, it is desirable to have a good barrier to oxygen or water vapor. However, the most common barrier materials, which are polymer based resins such as ethylene vinyl alcohol copolymer ("EVOH") or polyvinylidene chloride ("PVDC"), although exhibiting good barriers to oxygen or moisture, do so only under ideal conditions. Although EVOH can be an excellent oxygen barrier, it looses its barrier property at moderate to high relative humidity. Thus this material is not widely usable in applications involving high water vapor content.
Although PVDC exhibits good moisture and oxygen barrier properties, it is not suitable for many applications, and has an undesirable yellow color. Other proposed alternatives to provide oxygen and water vapor barriers include laminations of aluminum foil and aluminum metallized film. Although these exhibit good barrier properties, they are completely opaque, and in addition they cannot be readily used in environments involving microwaves, which may be desirable many times in applications involving both optical and electronic conditions.
U.S. Pat. No. 4,702,963 discloses packaging film in which an adhesion layer is first vacuum deposited on a flexible polymer substrate, followed by vacuum deposition of a barrier layer, to confer retortability to the packaging film. The adhesion layer can consist of Cr, which is preferred, co-deposited mixtures of Cr and SiO having at least 20% by weight Cr, among others. The barrier layer is preferably silicon monoxide or silicon dioxide. When silicon dioxide is used, it may be mixed with glass modifiers such as oxides of Mg, Ba, and Ca, or with fluoride of alkaline earth metals, e.g., MgF.sub.2. The glass modifiers serve to alter the color appearance of the overall coating. For example, a chromium/SiO composite film is disclosed to produce a coating with a yellowish appearance, while a neutral gray appearance is disclosed to result from the mixture of SiO.sub.2 with glass modifiers.
Japanese patent application 60-244540 discloses a laminate comprising the formation on the surface of a plastic film a transparent thin layer of one or more materials selected from metals, metal oxides, or glass by means of a dry plating method, providing a laminate with good barrier properties. Suitable metals include aluminum, silicon, iron, gold, silver, copper, chromium, nickel, tin, titanium, and magnesium. Suitable oxides may be the oxides of these metals (such as silicon oxide, which can be mixtures of silicon monoxide and silicon dioxide), and glass. A mixed evaporation or multilayer evaporation may be performed.
Japanese patent application 61-47244 discloses a laminate of a plastic film or sheet on the surface of which has been formed a transparent thin layer by dry plating one or more of the materials selected from metals, oxides of the metals, and glass. Suitable metals include aluminum, silicon, titanium, tin, iron, gold, silver, copper, chromium, nickel, magnesium, or the like. The oxides are those of these metals, or glass. These metals and metal oxides may be evaporated in a mixed state to form a layer or evaporated to form a multilayer. The laminate is said to have excellent gas-barrier performance.
U.S. Pat. No. 4,528,234 discloses a transparent laminate comprising a transparent plastic resin film substrate, a thin layer of at least one metal such as aluminum, tin, iron, zinc, or magnesium formed on the substrate by vacuum deposition, and a carboxyl group-containing polyolefin (e.g., ionomer) layer formed on the metal layer by lamination. Optionally an additional layer of silicon oxide or titanium oxide may be present. Oxygen and moisture impermeability are said to be improved.
U.S. Pat. No. 3,442,686 discloses a composite film structure suitable as a packaging film consisting of a flexible transparent organic base sheet, e.g., PET (polyethylene terephthalate) film, having a top coating of a polymer such as polyethylene and an intermediate, gas barrier, glassy coating of an inorganic material such as a silicon oxide. Other inorganic compositions which are useful include lead chloride, silver chloride, calcium silicate, and crushed "Alundum" (Al.sub.2 O.sub.3 with SiO.sub.2 binder).
Japanese patent application 62-158677 discloses a transparent laminate wrapping material where a thin single or mixed metal oxide layer is an intermediate layer in a laminate structure. The laminate is said to have excellent gaseous oxygen and water vapor barrier properties. Silicon oxide and aluminum oxide-silicon oxide mixtures are effective.
Japanese patent application 62-156943 discloses a vapor-deposited layer built-in type multilayered gas-barrier film or sheet having two or more vapor-deposited layers of metals or metal compounds formed at one or more laminate interfaces of a multilayered synthetic resin film or sheet, having good gas barrier characteristics. Suitable metals include aluminum, zinc, copper, platinum, indium, tin, gold, silver, and silicon. A suitable metal compound is silicon oxide.
Chahroudi, paper presented at annual technical meeting of Society of Vacuum Coaters discloses transparent barriers of silicon oxide. The SiO.sub.2 can be alloyed with oxides of various metals to improve barrier properties.
Sakamaki, paper presented at "Barrier Pak '89" discloses barrier properties of film with a thin layer of ceramic such as SiO in particular silicon oxide.
U.S. Pat. No. 3,522,080 discloses a process for hardening the surface of a synthetic material such as a lacquer film, which includes vapor deposition of layers of silicon oxide (SiOx derived from SiO.sub.2) onto the surface. The silicon oxide can contain 1.5 to 5 percent oxide of chromium, zinc, zirconium, or antimony.
U.K. patent application 2 197 881 discloses a heat resistant vessel made of a thermoplastic polyester resin by forming an inorganic coating layer comprising a silicon compound or a metal oxide-containing silicon compound on a surface of the polyester resin. The inorganic coating layer is obtainable from colloidal polysiloxane compounds. The coating material may further contain additives such as an inorganic filler of e.g., titanium oxide, zirconium silicate, nickel, copper oxide, manganese oxide, alumina, etc.
In certain of the above references, coatings of silicon monoxide (SiO), silicon dioxide (SiO.sub.2), or combinations thereof with a variety of metal oxides have been disclosed. There has been lacking, however, teaching as to the type and quantity of metal or metal oxide required to provide coatings of SiO.sub.2 with improved barrier properties. It has now been observed that combinations of SiO.sub.2 with many metals or metal oxides in fact do not provide improved barrier performance or alternatively reduce the optical transparency of films coated therewith to an objectionable extent. Furthermore, much of the relevant art focuses on SiO as the primary barrier layer. The use of SiO is not practical for many packaging applications because it is quite expensive and exhibits an objectionable yellow color. The present invention, in contrast, overcomes these shortcomings by providing an inexpensive inorganic coating with good barrier performance and good light and microwave transparency, suitable as a barrier coating for optical waveguide devices.